1. Field of the Invention
The present invention relates to an image forming apparatus that employs an ink jet system capable of forming an image by ejecting ink drops from a printing head mounted on a carriage onto a sheet.
2. Discussion of the Background Art
In a conventional image forming apparatus that employs an ink jet system (e.g. an inkjet printer), an image is formed by ejecting ink drops from a printing head mounted on a reciprocating carriage onto a sheet. Then, an inertial force is caused when the carriage is either accelerated or decelerated thereby vibrating an apparatus body. Especially, to increase a printing velocity, a velocity of the carriage is increased, and accordingly acceleration and deceleration become sharp. As a result, the apparatus body becomes largely vibrated and makes a user uncomfortable. Further, in a copier in which a scanner unit is mounted on a printer, vibration of the printer impacts scanning of a carriage and degrades image reading. Then, the below-described known countermeasures against the vibration have been taken, in which the object is to smooth out vibration profiles during times of acceleration and deceleration of the carriage. Alternatively, a notch filter or the like is used to filter some of a frequency band inputted to the apparatus, or a separate actuator is employed.
An apparatus generally has a prescribed natural vibration frequency in each of various modes, such as a deflection mode, a twisting mode, etc. Thus, the apparatus largely vibrates at the prescribed natural vibration frequency in response to an input having the same natural vibration frequency as the apparatus. The above-described notch filter prevents resonance of the apparatus at the natural vibration frequencies by avoiding an input having the same natural vibration frequency as that of the machine.
However, when a sheet moves up and down with respect to a carriage serving as a vibration source, a balance between mass of sheets ejected and stacked on a sheet ejection tray arranged on the top of the apparatus and that remaining in the sheet feeding tray arranged in the lower portion of the apparatus than the carriage changes significantly. In such a construction, a natural vibration frequency and a gravity center of the apparatus also change significantly in relation to the carriage. Further, since a natural vibration frequency also changes significantly when acceleration of the carriage is decreased or a notch filter is used, a band to avoid such an input becomes significantly wider, and acceleration of the carriage is late, so that both productivity and a width of the apparatus decrease. Similarly, when the other actuator damps vibration, and a natural vibration frequency and a gravity center of an apparatus change significantly, vibration-damping effectiveness is degraded.
In Japanese Patent Application Laid Open Numbers 2001-138499 and 2005-081673 (JP-2001-13849-A and JP-2005-081673-A, respectively), it is described that a vibration damper having the same mass as the carriage is attached to a timing belt that moves the carriage, and the carriage and the vibration damper are oppositely moved to damp the vibration of the carriage.
Further, in Japanese Patent Application Laid Open Number H3-256772 (JP-H03-256772-A), it is described that a scanning mechanism other than a scanning mechanism that moves a printing head is employed. The other scanning mechanism includes a weight having substantially the same mass as the printing head and is moved at the same acceleration as the printing head but in the opposite direction.
However, these conventional techniques do not effectively damp the vibration of the apparatus considering the up and down movement of a gravity center of the apparatus as well as a large change in natural frequency thereof when a sheet moves up and down with respect to the carriage serving as a vibration source as mentioned above.